Widespread use of DC defibrillators in patients suffering cardiac arrest has greatly increased the rate of successful resuscitation both in and out of hospitals over the past few decades. Defibrillation is applicable to life-threatening cardiac arrests resulting from ventricular fibrillation which occurs because of asynchronous depolarization of cardiac cells. When sufficient electrical energy is delivered to the heart from an external defibrillator through a set of paddles (electrodes), all cardiac cells briefly arrest and thereafter synchronous or normal depolarization may once again resume.
The defibrillator equipment presently offered to the medical arts (stores) discharges the electrical energy through an RLC circuit which is manually triggered by the physician. The heretofore standard quantity of the electrical pulse to be delivered has been calibrated in terms of joules of energy. The many studies reported in the medical literature of attempts to determine the optimal electrical strength of the pulse that should be delivered for defibrillation are almost invariably analyzed in terms of joules. Delivery of more than enough electrical energy for defibrillation has been associated with cardiac cell death, yet insufficient energy will not accomplish the desired defibrillation, resulting then in multiple attempts to defibrillate at ever higher energy levels.
Heretofore, recommendations for the "first attempt" defibrillation have been based on gross energy levels e.g., 100 joules. For example, U.S. Pat. No. 3,782,389 discloses a defibrillator that is computer controlled so that the energy actually delivered equals the predetermined energy sought to be delivered regardless of the patient load resistance. U.S. Pat. No. 3,862,639 adds the improvement of basing defibrillating current on the body weight of the patient.
The prior art approach to preselection of energy dose level for threshold defibrillation is believed to be suboptimal for several reasons. Body weight actually correlates poorly with threshold defibrillating requirements. For a given pulse duration, peak current is a better predictor of the defibrillation threshold than delivered energy. The inventor hereof has ascertained that defibrillating pulse levels based upon a specified level of peak amps per ohm of transthoracic resistance correlate well with threshold defibrillation requirements. Establishment of the defibrillation pulse on the basis of total electrical energy, as done by prior workers in the art, will not apply a consistent level of peak current (amperage) because transthoracic resistance, person to person, will vary within surprisingly large ranges.
The object of this invention is to provide a defibrillator method and apparatus for automatically providing a predetermined threshold level peak current according to the transthoracic resistance of each patient.
A further object of this invention is to provide a method and apparatus for determining and applying a threshold level of peak defibrillating current based on amps per ohm of transthoracic resistance.
Further objects of the invention and the advantages thereof will become apparent from the description which follows.